1. Field of the Invention
The present invention relates to a vaccine that protects against Marek's disease.
2. Description of Related Art
Marek's disease (MD) is a highly contagious neoplastic disease of domestic chicken that affects chickens worldwide and causes high mortality and condemnation if the chickens are not vaccinated at one day of age. MD is caused by a highly cell-associated oncogenic herpesvirus known as Marek's disease virus (MDV).
A number of live virus cell-associated vaccines are available that protect chickens against MD. These vaccines are maintained and administered in delicate cell-associated form. The vaccines need special handling and must be stored and transported in a frozen state in liquid nitrogen in order to maintain their viability and efficacy. These existing vaccines must be maintained and administered in cell-associated form, a condition that is costly and cumbersome.
The known vaccines contain the entire MDV genome, including sequences related to induction of pathogenesis. Although the existing vaccines against MD are either attenuated or are naturally apathogenic, viral mutation is known to occur in herpesviruses and there is a possibility that virulent pathogenic mutants may emerge in such vaccines. Such mutants could be less effective and even harmful.
Churchill et al, Nature, 221:744-747 (1969) and Okazaki et al, Arian Dis., 14:413-429 (1970) developed the first effective and safe vaccines against MD. These vaccines have been in use for the last 20 years and have reduced losses to the poultry industry worldwide. Other candidate vaccines based on serotype 2 naturally apathogenic MDV, Schat et al, J. Natl. Cancer Inst., 60, 1075-1082 (1978), or newly attenuated serotype 1 MDV, Rispens et al, Arian Dis., 16:108-125 (1972), and combinations of these viruses as bivalent vaccines, Witter, Arian Dis., 31:252-257 (1987), have helped provide a better protection against MD. All these vaccines, except the herpesvirus of turkeys (HVT) vaccine, require the storage and transportation in frozen state in liquid nitrogen and have to be administered as infected cells which calls for careful procedures to prevent inactivation of the vaccine. Even in the case of HVT vaccine, cell-associated viruses have been most widely used because they are more effective than cell-free virus in the presence of maternal antibodies, Witter et al, Avian Pathol., 8:145-156 (1978).
Recombinant DNA technology has allowed the construction of recombinant vaccines that contain only those desired viral genes or gene products that induce immunity without exposing the animal to genes that may induce pathological disorders. Pox viruses, including avipox virus, especially the fowlpox virus (FPV), provide excellent models for such vaccines. These viruses have a large DNA molecule with numerous non-essential regions that allow the insertion of several immunogenic genes into the same virus for the purpose of creating multivalent vaccines. These multivalent vaccines may induce cell-mediated as well as antibody-mediated immune response in a vaccinated host. Vaccinia virus (W) has been used extensively for this purpose and a number of VW recombinants have been constructed that express a variety of foreign genes including those that elicit neutralizing antibodies against glycoproteins of herpes simplex virus (HSV) type 1, Blacklaws et al, Virology, 177:727-736 (1990). Similarly, there are a number of reports describing the expression of foreign genes by recombinant FPV, Boyle et al, Virus Res., 10:343-356 (1988) and Ogawa et al, Vaccine, 8:486-490 (1990).
MDV homologues of the HSV gene coding for glycoproteins B, C, D, H, and I (gBh, gCh, gDh, gHh and gIh) have recently been cloned and sequenced, Coussens et al, J. Virol., 62:2373-2379 (1988), Ross et al, J. Gen. Virol., 70:1789-1804 (1989), Ross et al, J. Gen. Virol., 72:939-947 (1991), Ross et al, European Patent Application International Publication No. WO 90/02803 (1990).